1. Field of the Invention
The present invention generally relates to disk drives, and more specifically to a digital servo-control system for controlling a servo-head of a disk drive during a seek mode.
2. Description of the Related Art
Japanese Laid-Open Patent Specification Hei-3-30153 discloses a digital seek controller, in which target velocity trajectory is stored in a lookup table and a combined feedforward/feedback control scheme is provided for calculating acceleration and deceleration. In the prior art, the difference between a target velocity in the lookup table and an estimated velocity is detected and weighted by a proportional coefficient to produce a control signal, which is then used as a feedback signal to decrease the error between the estimated and reference values.
However, in the digital seek controller where all control data are sampled at clock intervals, off-track position data are obtained at discrete intervals and hence the system""s response tends to fluctuate when the head position is approaching a target track (i.e., xe2x80x9cseek-settling responsexe2x80x9d), resulting in a long seek time. This tendency is particularly acute in applications where control data are sampled at relatively long intervals.
In addition, if high speed seek control is desired, rapid acceleration and deceleration of servo-head would be necessary. However, due to the presence of possible variability among head/disc assemblies, errors at the rapid deceleration cannot be sufficiently absorbed by the combined feedforward/feedback approach and the servo-head speed near the target track fluctuates. Hence, settling time is prolonged.
It is therefore an object of the present invention to provide a seek control method and system that achieve smooth xe2x80x9cseek-settlingxe2x80x9d response.
Another object of the present invention is to ensure robust xe2x80x9cseek-settlingxe2x80x9d response even though the operating characteristics of the servo-head may differ from the standards due to manufacturing tolerances.
According to the present invention, there is provided a method of controlling the velocity of a servo-head on a recording disk during a seek mode, using a memory which defines relationships between a plurality of estimated remaining seek distance values of the servo-head to a target track on the disk and a plurality of corresponding target velocity values, an actuator for moving the servo-head, driving circuitry for processing an input signal to produce an analog drive current and energizing the actuator with the analog drive current, an analog-to-digital (A/D) converter for sampling the analog drive current and producing therefrom a digital sample of drive current during each sampling interval, and a head position detector for producing a head position signal indicating the position of the servo-head. The method comprises the steps of (a) during each sampling interval of the A/D converter, receiving a digital sample therefrom and a head position signal from the head position detector and estimating from the received digital sample and head position signal a one-sample future velocity value and a one-sample future remaining seek distance value, and setting the one-sample future velocity and remaining seek distance values as previous velocity and remaining seek distance values, (b) estimating a two-sample future velocity value and a remaining seek distance value from the previous velocity and remaining seek distance values using a calculated current value, (c) reading a target velocity value from the memory corresponding to the estimated two-sample future remaining seek distance value, (d) detecting a difference between the velocity value of step (b) and the target velocity value of step (c), and (e) deriving an incremental current value from the difference and summing the incremental current value with a previous current value to produce an accumulated current value. During the sampling interval, functions (b) to (e) are repeatedly performed by using the accumulated value of control current as the calculated control current value in order to decrease the difference. When the difference becomes smaller than a predetermined value, the accumulated control current value is supplied as a final value of the given sampling interval to the driving circuitry where it is processed and supplied to the actuator during the next sampling interval.